带返回和中断的开关盒

Question

Just out of curiosity, I often see situations like:

switch(something) {
    case 'alice':
        return something;
    break;
}

Where the break seems to be completely unnecessary, is there any reason for it to be there anyway?

Answer 1

The break; statement may have been there before the return statement was introduced. As such, it has become redundant and can be removed.

In fact, when you run that code through jslint , it will show this error:

Unreachable 'break' after 'return'.

Whether or not to heed this advice is up to you; it could be helpful during development if you're trying out a few things before settling on a particular style.

This is an alternative writing style that some might argue is a better practice:

var retval = null;
switch (something) {
    case 'alice':
        retval = something;
        break;
    // ...
}
return retval;

Answer 2

break tells javascript to stop evaluating cases in the switch block. Code execution continues past the closing switch bracket. The return statement in the example code will indeed prevent further of anything past it, including other case statements and anything following the switch block.

I put a break statement in every case by habit. If I wrote a case without a break then I might copy and paste blocks of code around in the future and the lack of a break statement would become a bug like so:

function whereLivesA(species){
  switch(species){
    case 'worms': 
      // Relying on return to prevent further code execution within the switch
      // block works but is ~bad~ smelly (according to plato :D)
      var habitat = 'dirt'
      return (species + ' live in ' + habitat);
    case 'bees':
      var habitat = 'hive';
      break;
  }
  // Stuff to do after the switch statement (unless you returned already)
  var str = species+' live in '+habitat;
  return str;
}
console.log('whereLivesA');
console.log(whereLivesA("worms"));
console.log(whereLivesA("bees"));
  /* Output:
    whereLivesA
    worms live in dirt
    bees live in hive
  */


function whereLivesB(species){
  switch(species){
    case "worms": 
      // what if future code changes remove `return` and don't add `break`?
      // return (species + ' live in ' + habitat)
      var habitat = 'dirt';
      // break;
    case "bees":
      var habitat = 'hive'
      break;
  }
  // Stuff to do after the switch statement (unless you returned already)
  var str = species+' live in '+habitat;
  return str;
}
console.log('whereLivesB');
console.log(whereLivesB("bees"));
console.log(whereLivesB("worms"));
  /* Output:
    whereLivesB
    bees live in hive
    worms live in hive
  */


function whereLivesCorrect(species){
  switch(species){
    case "worms": 
      var habitat = 'dirt';
      break;
    case "bees":
      var habitat = 'hive'
      break;
  }
  // Stuff to do after the switch statement (unless you returned already)
  var str = species+' live in '+habitat;
  return str;
}

console.log('whereLivesCorrect');
console.log(whereLivesCorrect("bees"));
console.log(whereLivesCorrect("worms"));
  /* Output:
    whereLivesCorrect
    bees live in hive
    worms live in dirt
  */

JS newbies: If you don't want to save it to a file and run node filename , you can press F12 and paste this script or other self contained scripts into your browser's console to run it.

If you use node.js you can also type node at a command line to start a node console and paste it there.

Answer 3

The break keyword is used to end the statement, or exit out of the loop so it doesn't continue running.

for example:

html

what's your age?: <input type="text" id="ageOf"><br>
<input type="submit" onSubmit="postReply();">
<div id="reply"></div>

js

function postReply() {
  let ageOf = document.getElementById('ageOf');
  let response = document.getElementById('reply');

  switch(true) {

    case ageOf<18:
      response.innerHTML = "Keep practicing young padewan.";
      break;

    case ageOf>18 && ageOf<45: 
      response.innerHTML = "Much too learn, you have.";
      break;

    case ageOf >= 45:
      response.innerHTML = "You have sage wisdom.";
      break;

    default:
      response.innerHTML = "";
      break;
  }
}

so on submission, the form should call the function postReply() , check the user answer and depending on the value, it should return one of those statements.

Answer 4

the case is actually very necessary

You have a case, then you need to break from that case or else the other cases are picked up as well.

using cases is most often considered bad practice, stay away from them as much as possible.

switch(casein){
case 1:{
break;
}
case 2:{
break
}
}